FLASH TANK

If we say that Chemical Engineering is nothing but the combination of art and science to design and control the separation equipment, it won’t be a lie. In a chemical industry, more than the 70% of total capital investment is incurred on separation and purification equipment. These stats might highlight the importance of separation equipment in chemical industry.

Defining the problem:-

                  

In Cativa Process, one of the product streams is coming out from the reactor. This stream contains the Acetic Acid; which is our sole product, and the Iridium Catalyst Complex. We have to maintain some liquid level in the reactor as well so that we might use this liquid as the solvent for the incoming feed stream. The catalyst has to be recycled back to reactor for further utilization. So we need equipment that might separate out the product (not essentially all of it) and recycle back some fraction of Acetic Acid along with the catalyst. A little amount of water should also be maintained in the reactor as this is the requirement of the technology (Cativa Process) we are using. So up till now, we have successfully defined our problem. Let’s look for a solution to it.

Looking for the solution:-

                  

                   Now there are a number of equipments that are available to us for this purpose. We need to have a look at the physical conditions of the stream. All the components are in liquid state at 110 ^oC and 27 atm pressure.

We need to recycle some of the Acetic Acid and the catalyst back to reactor. Both of these are required to be there for further conversion. The feed mixture is in homogenous phase. This makes our choice quite simple. We can eliminate the possibility of a phase separator. One thing that must be kept in mind is that the solution has to be economical and quite effective. If we have a look at various industries; we find that most industries generate a second phase from this feed and recycle successfully some of the desired components in liquid state. This is quite an energy efficient process. Now let’s have a look at the possible choices that we have at our hand.

Possible Choices Available:-

                       

We have our feed in liquid state in which catalyst is homogenously dissolved. We want some of the Acetic Acid, little amount of water and the catalyst recycled back to reactor. We’ll make use of equipment that can generate the vapor phase without expenditure of much of external energy and then successfully recycle the desired components back to reactor. One choice looks obvious. It’s the Flash Drum. There are other possible alternatives available to us, likewise Knockout Drum, Horizontal Flash Drum or the spherical one. All have their own characteristics and are used in specific situations. We’ll make use of Vertical Flash Drum.

Construction of a Flash Drum:-

                When feed is flashed in a Flash Drum, vapor and liquid mixture is generated. As this mixture enters the drum, the surface area is increased, due to which pressure drop is generated. Right at eh entrance of the feed, there’s a splash plate in the drum. This splash plate directs the vapor and liquid flow downwards. This way the effect of gravity is enhanced. The liquid settles down at the bottom while the vapors with little momentum, change their path and rise up the vessel. At the top of the vessel, there’s a mist eliminator. Actually when vapors rise up the vessel, small liquid droplets also accompany them. The phenomenon of splashing is avoided by the use of splash plate. So our splash plate is serving two major purposes. First it helps us to avoid the splashing of liquid. Secondly, it directs the vapor liquid mixture downwards which in turn enhances the effect of gravity. Due to this effect, liquid is separated out of vapor. One thing should be kept in mind is that most of the impaction process takes place at the splash plate. So it has to be mechanically sound so that it can handle all the impact. Now there are two kinds of mist eliminators.

• Vane type Mist Eliminator
• Mesh Eliminators

Vane type mist eliminator consists of metallic plates arranged closely to each other. Vapors with small liquid droplets rise. The plates are arranged in such a manner that they provide a zigzag path to the incoming vapor and liquid droplets. Droplets due to inertia and large momentum strike the plates and are captured at the surface while the vapors change their path accordingly and escape the eliminator. The phenomenon is referred to as Impaction and the size increase of droplets is called as Coalescence. Hence vapors are collected at the top of the vessel.

Now in mesh mist eliminators, a metallic or plastic wire mesh with a diameter ranging 0.006 to 0.011 in is used. The phenomenon is the same; impaction on the wire and then captured. Mist escapes the wire while droplets are captured at the surface where they coalesce and fall down as large drops.

There’s a radial vane vortex breaker shown at the bottom of the vessel. The purpose of this vortex breaker is to avoid the phenomenon of Vortex Formation. There are a couple of causes that induce the vortex formation in the drum. The first one is the earth’s rotational speed. Due to the earth’s rotational speed, anticlockwise vortex is observed in Northern Hemisphere while a clockwise motion is observed in Southern Hemisphere. Second reason is the introduction of feed in the vessel tangentially. Whenever feed is entered tangentially, vortexes are formed. Third reason is the vapors. Whenever there’s a two phase mixture and they differ in their velocity; then the fluid with lesser velocity and high density would start the rotational motion (Vortex Formation). In our case, we are handling a vapor-liquid mixture. Vapors are at a higher speed in the vessel while the liquid are a bit slower due to the impaction with the splash plate. So the vapors would induce the vortex to the liquid. The formation of vortexes brings some disadvantages to the system. Our system with vortex formed, experiences:

• Loss of valuable vapors
• Downstream equipment damage
• Loss of flow
• Erroneous liquid level readings resulting in poor control
• Vibrations caused by unsteady two phase flow.

The formation of vortexes is shown in the following figure:

To avoid the vortex formation, we should avoid the usage of a tangential feed line. Secondly, we can use a vortex breaker to get rid of vortexes. Following types of vortex breaker are usually used in the industry:

• Flat plate vortex breaker
• Crosses
• Radial vane or gratings

We are using a Radial Vane Vortex Breaker. A vortex breaker is stationary and it doesn’t move. If it starts the motion with the vortex then it wouldn’t break the vortex rather it would just weaken it. To break the vortex and get rid of it, we’ll have to fix the vortex breaker and make it stationary.

Why use Vertical Flash Drum?

               

Let’s carry out the process of elimination to justify our choice. We can simply rub aside the choice of Knockout Drum as it is used wherever there’s gas in the feed stream. In our stream there are no gases. We have only liquid phase. So we will not go for the Knockout Drum. Now we are left with Horizontal, Spherical and Vertical Flash Drums. Horizontal Drums are used when we have to handle a large liquid flow rate. But in our case we’ll see that the liquid flow rates wouldn’t be that huge. Instead we’ll have to deal with a high amount of vapor flow rate. Also Walas carried out a survey and in his book “Chemical Process Equipment Selection and Design” writes that out of every ten chemical industries; seven are making use of Vertical Flash Drums. The choice is made due to the economy and the ease with which we can handle the flow rates. A design engineer is required to start designing a Vertical Flash Drum by default and then after the design is complete we have a look at the L/D (length to Diameter Ratio) to decide which configuration to use. So we’ll follow the same procedure. We’ll design a Vertical Flash Tank and then would analyze the L/D ratio obtained to determine which configuration to use. Just remember one rule of thumb; for large liquid flow rates, we’ll use Horizontal Flash Drum and for small liquid flow rate, you’ll go for a vertical configuration. You can start designing any one of these and then the final decision would rest upon the L/D ratio of the drum. So don’t bother. Just start your computer software and begin designing any configuration. Let’s start the design of Vertical Flash Drum. Before the process of designing, we’ll see what exactly flashing is.

Throttling:-

               

When a fluid (liquid or a liq/vapor mixture) at high temperature and high pressure experiences sudden reduction in pressure, then some of the liquid is vaporized and the phenomenon is referred to as Throttling. During the process the temperature of the feed stream doesn’t change that much and in such a case the process is called as Adiabatic Flashing. Actually for an ideal gas or a fluid behaving likewise an ideal gas, there’s no temperature drop. But in real fluids, little temperature drops have been observed. These temperature drops are due to the Joule-Thomson Effect and the frictional loss. Since there’s no appreciable change in the kinetic and potential energy; and also there’s no shaft work or heat transferred, therefore the eq:        

Δ (H + u²/2 + gz) = Q + W_s reduces to ΔH = 0.

                        We know that the enthalpy depends upon the temperature of fluids. Since there’s no change in enthalpy so theoretically there will be no change in the temperature of the fluid stream. Usually for real fluids, a very little temperature drop is observed. In our case, the feed is at 110 ^oC and the pressure is 27 atm. We’ll suddenly reduce the pressure of the liquid stream and this would ultimately generate a vapor phase without the expenditure of any external energy. There will be ignorable temperature drop. All the beauty of equipment lies in this phenomenon. We are generating a second phase without expanding any external energy. But we know that energy is always conserved. We have generated the vapors on the expense of the pressure of the incoming feed So although the process of throttling makes us lose some of the energy contents of the feed stream, yet we get more benefits. Now the problem comes out to be the selection of the valve.

Selection of Valve:-

No ordinary valve would be used for this purpose. We need such a valve that would handle a feed stream with such a high temperature and pressure and allow it to expand suddenly. The valve would allow only one sided flow of the stream. There are a number of options open to us. Globe Valve, Gate Valve, Butterfly Valve, Ball Valve etc are all at our disposal. But none of these is manufactured for the purpose of throttling. As we look for the best choice, we come to know that there’s a valve that is manufactured keeping in mind the sole idea of throttling. This is Lever sealed Plug Cock. The valve operates up to a temperature of 260 ^oC. It has plastic lining that makes it corrosion resistant. It has a tapered plug that is moved by a lever. The plug contains perforations just like a ball valve. As the feed stream passes through it, pressure drops from 27 atm to 1.4 atm. The temperature change is negligible. So after getting flashed, vapors are generated. The temperature of the stream remains more or less the same.